1. Field of the Invention
The invention relates to position indicators in general and more specifically to a highly accurate electronic position indicator utilizing a linear voltage differential transformer (LVDT) in combination with accuracy assurance circuitry.
2. Description of the Prior Art
There are numerous applications for position indicators. An example of one such application is in pressure sensing systems. Prior art position indicators employed in pressure sensing systems may be used to exemplify the problems inherent in prior art position indicators in general. Thus, for the purpose of illustration only, the discussion immediately following will focus on position indicators for pressure sensing systems.
Pressure sensing systems are known which employ a differential pressure transducer or a similar device to provide a mechanical movement which is a function of the pressure being measured. This mechanical movement is typically converted via mechanical means such as gears and levers or servo mechanisms to drive a meter calibrated in units of pressure. Using these devices for remote measuring of pressure, however, does give rise to certain problems. In the case of an entirely mechanical pressure sensing gage, accurate indication of pressure at a remote location requires long mechanical linkages or a fluid carrying line brought to the remote location of the gage. This approach, while accurate, is quite expensive to implement.
In the case of the servo mechanism approach, expense becomes a problem because of the high cost of servo mechanisms. In addition, servo mechanisms may be physically larger than is desired for a given application, such as in aircraft, and they usually require more electrical power than is desirable.
In attempting to overcome some of these difficulties, position indicators employing a LVDT have been used. These position indicators include a mechanical linkage between the movable core of the LVDT and a mechanical transducer whose movement is controlled by the item being measured, for example, pressure. The movement of the core is translated into a variable voltage at the LVDT tap and this voltage is used to indicate the core position. While this approach is considerably simpler and less costly than the above mentioned position indicators, the accuracy of the simple LVDT approach is often unacceptable. This is because the tap voltage is a direct function of the voltage applied across the LVDT and, therefore, is subject to variation should the supply voltage vary.
Other circuits have been developed in an attempt to provide accurate position indicators with a LVDT which are independent of the input voltage changes. One such approach employs a buckling coil on the transformer so as to minimize the effect of line voltage changes. This approach, however, wastes a great deal of power and, therefore, is not desirable. Other circuits utilizing reactive components have been developed. The accuracy of these circuits however, is dependent on maintaining a constant operating frequency on the AC power lines to the circuit. In certain applications, especially in aircraft, the on board AC power supply frequency is nominally 400 cycles but the actual frequency varies between about 380 and 420 cycles. As such, position indicators using circuits with reactive components have not been able to provide the desired accuracy in applications whose AC power frequency is subject to change.
In view of the foregoing difficulties associated with the prior art, it is an object of the invention to provide a position indicator which is substantially unaffected by changes in input voltage or frequency.
It is a further object of the invention to provide a highly accurate position indicator which may be operated with a minimum of power consumption and which occupies a minimal amount of physical space.
It is still a further object of the invention to provide a position indicator whose accuracy is substantially unaffected by changes in input voltage or frequency at a cost which compares favorably to known accurate approaches.